Fix 'make dist'

Fix 'make dist'

EXTRA_DIST copies the listed directories/files from the _source_ directory
into the distribution.

Since the doc directory does not exist after running autogen + configure
+ make dist, the distribution tarball generation fails.

Note that the dist-hook rule below operates on 'distdir', not on the source
directory where EXTRA_DIST expects the existence of the doc folder.

In summary, even if I removed 'doc' from EXTRA_DIST, the dist tarball will
always contain the documentation (due to the dist-hook rule).

Signed-off-by: László Várady <laszlo.varady@balabit.com>